Harbour porpoises (Phocoena p. phocoena L.) are vocal animals and their activity can be monitored
effectively using underwater, autonomous, passive-acoustic cetacean-click detectors called T-PODs
[e.g. 1, 2, 3].
The characteristics of porpoise-echolocation clicks have been described in great depth over the last
forty years [4-10]; clicks can be emitted singularly or in groups known as “trains”. There is a linear
correlation between porpoise-echolocation pulse intervals and target range [11, 12] with a peak in
repetition rate as the animal nears the target, analogous to the “terminal buzzes” repeatedly
observed in echolocating bats [13]. Determination of a successful prey-capture event in wild
echolocating bats has been achieved effectively [e.g. 14] but for wild porpoises, underwater filming
of prey-capture attempts is extremely tedious. Moreover, in the wild, without visual confirmation, any
correlation between porpoise buzz activity and feeding success cannot be assumed a priori without
experimental evidence, because a high buzz rate may simply be associated with increased foraging
effort for the same amount of prey. Nonetheless, it is conceivable that by using acoustics alone, a
proxy of feeding activity could be surmised by examining the relative incidence of increasing click
rates, emitted during range-locking echolocation behaviour, and the associated decreasing interval
between clicks, known as “inter-click-intervals (ICI)” [see 2]. A link between feeding and decreasing
ICI has been established for foraging Blainville's beaked whales, Mesoplodon densirostris [15] and
harbour porpoises [16].